1. Field of the Invention
The present invention relates to a position measuring device, and to a method for operating a position measuring device.
2. Description of the Related Art
A differentiation is made in connection with position measuring devices between incremental and absolute position measuring devices. With incremental position measuring devices, two analog or rectangular scanning signals, which are offset from each other by 90°, are present at the output, from which a measured position value is only formed in a follow-up electronic device. With absolute position measuring devices, an absolute measured position value is already present at the output and can be provided to a follow-up electronic device.
With both categories of position measuring devices, the dependability and functional status depends on the quality of the scanning signals. It is possible in connection with incremental position measuring devices which output analog scanning signals to directly test the quality of the scanning signals in that these are supplied to both channels of a dual-trace oscilloscope, so that a Lissajous figure is generated on the screen. The radius of the Lissajous figure is a measurement of the amplitudes, as well as the phase relationship of both scanning signals. This procedure is described in WO 90/02956 A1 and JP 02 036313 A.
A direct measurement is not possible with incremental position measuring devices, which issue two rectangular scanning signals which are phase-shifted by 90° with respect to each other. In JP 08 029197 A it is proposed to provide a selector switch in the position measuring device, so that either the rectangular scanning signals, or the analog scanning signals which are phase-shifted by 90° with respect to each other, can be output on two data lines. During measuring operations the rectangular scanning signals are transmitted, and the analog scanning signals for error diagnosis.
In accordance with EP 0 962 748 A1 and DE 199 11 774 A1, status information of the position measuring device is transmitted analogously superimposed on the incremental rectangular scanning signals. In this case the decoding of the superimposed signals is relatively difficult.
Absolute position measuring devices are described in DE 44 22 056 A1 and DE 43 42 377 A1. In accordance with DE 44 22 056 A1, several scanning signals of a periodic measuring graduation, which are phase-shifted with respect to each other, are provided to a counter for creating an absolute position. This absolute position is bit-serially transmitted by the position measuring device as a multi-digit code word to an electronic follow-up device on a data line. Monitoring circuits are additionally provided in the position measuring device which detect the functional status of the position measuring device and, as a function of this, provide abnormality information via the data line. The temperature and the voltage supply are here cited as monitoring options.
In accordance with DE 43 42 377 A1, besides the absolute position, alarm and warning reports are also serially transmitted on a data line to an electronic follow-up device. Monitoring of the functional status takes place in the position measuring device.